1. Field of the Invention
The present invention relates to a treatment method for brine (salt water), and particularly to a brine treatment method for removing sulfate ions which are contained in brine containing alkali raw material such as sodium chloride or the like, or alkaline earth metal chloride, and at the same time withdrawing chloride ions to suppress loss of the alkali raw material or alkaline earth metal chloride.
2. Description of Related Art
In a process of electrolyzing brine which is solution of alkali or alkaline earth metal chloride to thereby produce chlorine, hydrogen and alkaline metal hydroxide, it has been known that sulfate ions which are contaminated as impurities in alkali raw material or alkaline earth metal chloride adversely affects an electrolysis capability, and thus removal of the sulfate ions has been performed from brine in this process. As a conventional method of removing sulfate ions from brine have been utilized various methods such as a method of removing sulfate ions as barium salt (barium sulfate), calcium salt (calcium sulfate) or the like, a refrigerating method and a salt-water purge method. However, when sulfate ions are removed with high efficiency by these methods, various problems such as cost-up, etc. occur. In order to improve the industrial efficiency of the sulfate ion removing process, various proposals have been recently made to remove sulfate ions using an ion exchange treatment. For example, these proposals are disclosed in Japanese Laid-open Patent Application NO. Sho-60-44056, Japanese Laid-open Patent Application No. Sho-60-228691, Japanese Laid-open Patent Application No. Hei-3-153522 (U.S. Pat. No. 5,071,563), Japanese Laid-open Patent Application No. Hei-4-300652, U.S. Pat. No. 4,415,677, U.S. Pat. No. 4,415,678, etc.
In the method as proposed in Japanese Laid-open Patent Application No. Sho-60-228691, brine containing sulfate ions is diluted, and then passed through anion exchange resin beads or powder so that the sulfate ions are adsorbed by the anion exchange resins and thus removed from the brine. Thereafter, the anion exchange resin in which the sulfate ions have been adsorbed is washed with dense brine to cause the sulfate ions to be adsorbed by the anion exchange resin and remove the sulfate ions. In this method, sulfate ions contained in dilute brine which is emitted from an electrolytic cell are adsorbed and condensed by ion exchange resins, and it is applied to the conventional calcium method. Therefore, the cost of this method is higher than the conventional method, and thus there is a problem industrially.
Furthermore, in the methods as proposed in Japanese Laid-open Patent Application No. Sho-60-44056, U.S. Pat. No. 4,415,677 and U.S. Pat. No. 4,415,678, macroporous cation exchange resins carrying amorphous polymeric or polymeric zirconium hydrous oxide is filled in a packed column, and then salt-water is allowed to flow through the packed column in a fixed state to remove sulfate ions. In this method, since an adsorbing treatment is carried out in a fixed bed, polymeric zirconium hydrous oxide which has already adsorbed sulfate ions comes into contact with brine of high acidity in the neighborhood of a salt-water inlet part of the packed column. Therefore, the polymeric zirconium hydrous oxide is dissolved in the acid, and thus a large amount of polymeric zirconium hydrous oxide is lost, so that a maintenance cost may be increased. When the adsorption treatment is performed with a high pH value to prevent dissolution of zirconium, a removing rate of sulfate ions is reduced, and thus it is not efficient. In addition, since the adsorbed sulfate ions are desorbed from the polymeric zirconium hydrous oxide with water, a regeneration efficiency is low and both sodium sulfate and alkali raw material or alkaline earth metal chloride are desorbed, so that loss of raw material chloride is great. Therefore, this method is ineffective economically. In addition, this method is unsuitable for such a present situation that restriction is imposed on discharge of chloride ions. On the other hand, when alkali is used for the desorption treatment, the desorption efficiency is high, however, aqueous solution having a high pit value and polymeric zirconium hydrous oxide are contacted with each other, so that there occurs a cation ion exchange reaction in which polymeric zirconium hydrous oxide adsorbs sodium ions. Therefore, acid or alkali whose amount is twice as large as a theoretical amount required to remove sulfate ions must be used, and this is industrially unfavorable.
Further, in the methods as proposed in Japanese Laid-open Patent Application No. Hei-3-153522 and Japanese Laid-open Patent Application No. Hei-4-300652, zirconium hydrous oxide powder containing a small amount of water is directly used, and it is contacted with sulfate-ion contained brine in a slurry state to adsorb and remove sulfate ions. The zirconium hydrous oxide which adsorbs the sulfate ions is contacted with alkali in aqueous solution to be regenerated. This method is more excellent than the method proposed in Japanese Laid-open Patent Application No. Sho-60-44056, etc. because the various problems as described above, such as the low regeneration efficiency, the increase of an amount of acid or alkali to be used and the loss of polymeric zirconium hydrous oxide in the adsorption process of sulfate ions, can be overcome by this method. However, this method needs to separate zirconium hydrous oxide, and it has new problems, for example, it needs a facility cost for a separation device such as a filter, etc., a maintenance cost, operators, etc. Furthermore, zirconium hydrous oxide powder and brine are contacted with each other in a slurry state by stirring or pump-circulation, so that the zirconium hydrous oxide powder intensively impinges on a stirrer or an impeller of a pump. Therefore, the powder may be ground or crushed to be pulverized. In this case, the pulverized zirconium hydrous oxide may be discharged to the outside of a system, and thus the loss amount is increased. Still furthermore, there occurs little adsorption of raw material chloride because no ion exchange resin is used, however, the raw material chloride may be discharged together with sulfate ions at the desorption time like Japanese Laid-open Patent Application No. Sho-60-44056.